Imagine the speed you would reach after going through the upper rapids of the Niagara River and then plunging over the edge of the Falls. Then imagine the force of 600,000 gallons of water per second (2,271,247 liters per second) crashing on top of you, not to mention the rocks you're bouncing around on under all of that water. The description that the surviving daredevils gave of what it was like when they went over the edge varied from "like a skydiver's free fall" to sheer terror. But of course, it's not the fall that'll get you; it's the landing.
First off, there is the danger you face inside the barrel itself (if you ride in one, that is). When you go over the 170-foot Horseshoe Falls, there is still another 180 feet to the Maid of the Mist pool. The forces that can knock you around inside the barrel are massively strong, so the chance of concussion-type injuries from rapid deceleration and acceleration is extremely high.
There is also the danger from hitting the rocks and having the barrel break apart. If that happens, you have no protection and will likely either drown or be battered to death in the currents and on the rocks. Then there is the danger of getting caught behind the curtain of water and running out of air before you can be rescued.
Of course, there is room for many other potential dangers, and many depend on the type of craft in which you go over.